Modem, high speed, automated concrete block plants typically make use of molds that are configured to produce multiple blocks simultaneously. These molds contain multiple mold cavities, where each cavity is typically open at the top and at the bottom. The molds are mounted in machines which cyclically station a pallet below the mold to close the bottom of the mold cavities, deliver dry cast concrete into the mold cavities through the open top of the mold cavities, densify and compact the concrete by a combination of vibration and pressure, and strip the mold by a relative vertical movement of the mold and the pallet.
There is a demand for a concrete block that, when laid up into a wall or other structure with other blocks, has an exposed face that has a natural appearance so that a resulting wall constructed from a plurality of the blocks appears to have been constructed with naturally-occurring, rather than man-made, materials.
Known methods for producing block faces with a natural appearance is by the splitting process described in U.S. Pat. No. 5,827,015, or by the splitting process described in U.S. Pat. No. 6,321,740.
Another method for achieving a block face that has a more natural appearance than is achievable by known splitting processes is disclosed in U.S. Patent Application Publication No. 2003/0126821. As disclosed in Publication No. 2003/0126821, a mold is provided that has a plurality of block cavities arranged in a row, with each cavity being configured to produce a concrete block with the block oriented with its front face facing upward in the cavity. Pressure plates, also known as a “stripper shoes,” are connected to a compression head. The pressure plates, which have a predetermined three-dimensional pattern formed therein, are pressed into dry cast concrete within the mold cavities by the compression head to densify the concrete and impart the patterns to the front faces of the blocks being formed in the cavities.
When imparting three-dimensional patterns to the faces of the blocks using pressure plates, as in Publication No. 2003/0126821, it is important that the compression head and the pressure plates connected thereto apply sufficient compaction pressure on the concrete in order to adequately densify the concrete. Insufficient densification of the concrete can result in a block that does not have the expected and desired strength properties. In addition, it is important that the compression head and pressure plates be level so they come down straight and contact the dry cast concrete in the mold cavities with even pressure across all of the cavities. If the head and the pressure plates are not level, uneven pressures may be exerted on the concrete in the cavities, thereby resulting in insufficient or uneven densification of the concrete in one or more of the cavities. Uneven densification can result in a block having strength properties that vary greatly across the block.
Further, insufficient and uneven densification can result in block-to-block variations in the dimensions of the blocks, particularly the depth of the block between the front and rear face of each block. Variability in the depths of the blocks can cause a wall or other structure built with the blocks to appear to be poorly constructed and to be visually unattractive.
There is a need for controlling the quality of blocks that are produced front face-up in a mold, to ensure that the blocks are manufactured with consistent quality and minimal block-to-block variability.